Separation of propylene (propene) from propane is well-known, as described in Zimmermann, H. and Walzl, R., 2009, Ethylene. Ullmann's Encyclopedia of Industrial Chemistry. Propylene fractionation separates propylene as a chemical-grade overhead product (typically 93-95 wt % min.) or more frequently as polymer-grade propylene (≥98 wt %) from propane. Separation to polymer grade propylene requires typically 150-230 stages and a reflux ratio of 20 because of the close boiling points of propylene and propane. One of the basic processes applied for this difficult separation task is operating polymer-grade fractionators at ca 1800 kPa, with cooling water in the overhead condenser and hot quench water in the reboiler. In the case of naphtha cracking and where sufficient waste heat is available from the hot quench water cycle this is the most economic process.
The condenser should operate at the lowest pressure that can be achieved by the cooling water, since a lower pressure leads to lowering of the cost. The cooling water typically has a temperature of 20-30° C. An example of a known separation system for separating a feed stream comprising propylene and propane is illustrated in FIG. 1.
In the example of FIG. 1, 20 t/h of liquid C3 product 101, containing 5% wt propane and 95% wt propylene, is fed to stage 78 of a distillation column C-101, which has 160 stages and a diameter of 4 meter. The pressure drop over the column C-101 is 1.3 bar. A reboiler H-101 has a duty of 18.8 MWth and produces 235 t/h of vapor 103. The distillation column C-101 produces 215 t/h of vapor 104 at the top that is condensed against cooling water in a condenser H-102, which is sent to vessel V-101. In the vessel V-101, 196 t/h of the condensed vapor is pumped back as a reflux 109 and 19 t/h of 99% pure propylene is produced as a product stream 111. The heat from the condenser H-102 is rejected to cooling water having a temperature of 20-30° C. In this case, the condenser H-102 operates at a pressure of 16 bara. The column is operated at a vapor velocity of 79% of the flooding velocity. The high pressure of the condenser H-102 makes the distillation more difficult, requiring a large amount of reflux. To accommodate the high vapor and liquid flows in the column as a result of the large amount of reflux, the column diameter needs to be large in order to avoid flooding of the column.
Thus, for operating at a vapor velocity of 79% of the flooding velocity using cooling water of 20-30° C. as in this example, the condenser pressure becomes 16 bara, which limits the amount of the liquid C3 product 101 fed to stage 78 of the distillation column C-101 to 20 t/h.
One known way of decreasing the pressure of the condenser is the use of a compressor for the vapor from the distillation column. FIG. 2 illustrates a separation system for separating a feed stream comprising propylene and propane, wherein a compressor is used. In the example of FIG. 2, the vapor 204 comes out from the distillation column C-201 at 9 bara and is compressed by K-201 to obtain a compressed vapor 205 of 14 bara. This compressed vapor 205 is fed to a heat exchanger H-201. In the heat exchanger H-201, the compressed vapor 205 provides heat to reboil liquid 202 from the distillation column C-201 to obtain 203. The compressed vapor 205 is condensed to obtain a stream 206. The stream 206 is then sent to a vessel V-201, where one part 208 of the stream 206 is pumped back as a reflux and one part 210 of the stream 206 is taken from the system as the propylene product stream. The disadvantage of this system is that it requires a compressor to work. The compressor requires high value energy, such as electricity (motor drive), or high pressure stream (steam turbine drive) to function.